What's clear: Katherine Russell claims she was completely in the dark about her husband's alleged plan to bomb the Boston Marathon on April 15.

WCVB

What's clear: Katherine Russell claims she was completely in the dark about her husband's alleged plan to bomb the Boston Marathon on April 15.

What's unclear: How could she not know?

Russell and Tamerlan Tsarnaev were married on June 21, 2010. She was raised in a Christian household in North Kingstown, Rhode Island, but converted to Islam after meeting her husband while attending Suffolk University.

She worked as a home health aide and would often leave their toddler daughter with her husband while she went to work.

"The reports of involvement by her husband and brother-in-law came as an absolute shock ..." Russell's lawyer said in a statement. "As a mother, a sister, a daughter, a wife, Katie deeply mourns the pain and loss to innocent victims -- students, law enforcement, families and our community."

There are plenty of open questions surrounding the Tsarnaev brothers' alleged terrorist plot, but this one is glaring: How did Katherine Russell miss signs that her husband was poised to dive off the deep end?

When we think "double life," we think about affairs, tax evasion, even pedophilia and rape, says Dr. Gail Saltz, the author of "Anatomy of a Secret Life: The Psychology of Living a Lie," but in the United States a terror plot seems relatively far-fetched.

Psychology experts say that while the signs vary depending on the nature of the "secret life," there are clear markers to look for in a loved one who might be harboring violent or harmful tendencies.

While there is still much to be learned about the transformation of Tamerlan Tsarnaev from aspiring Olympic boxer to apparent jihadist, there were some indications of instability.

Tsarnaev was apparently a follower of extreme religious teachings, did not assimilate into his adopted country and had been previously arrested and accused of domestic abuse (it is not clear whether the complainant was Katherine Russell or another woman.)

"He was somebody who was not able to make an adjustment," Saltz says.

Friends of Russell describe her husband as "controlling," "manipulative" and "combative." She began dressing very conservatively after converting, including the hijab or traditional covering for the hair and neck of Muslim women.

If Tsarnaev was combative in front of friends, Saltz says, he was likely even more confrontational in the privacy of their home. Physical or verbal abuse can combine with other signs to indicate that a spouse is interacting with the world in unhealthy ways.

A spouse or relative who is attempting to hide behaviors might create physical and emotional distance, have unexpected outbursts or opinions, or squirrel away money and spend it without consent.

People in relationships are entitled to do things on their own, according to Saltz, but a secret life is "one that harms the people they love."

While no one size fits all, Lisa Brateman, a New York-based psychotherapist and relationship specialist, says characteristics of people in denial about their partner's true self include: low self-esteem, rationalization, a sense of helplessness, and being passive, submissive and dependent.

Ruth Madoff, whose husband is now serving a 150-year prison sentence for perpetrating the largest financial fraud in history, stood by him during the trial and even tried to commit suicide with him after his $65 billion corruption was exposed.

Ponzi schemer Bernie Madoff was affable and charismatic on Wall Street, but in private he was intensely attuned to the goings-on at his 17th floor offices in Manhattan's Lipstick Building (though he had offices elsewhere, the location stored many of his money-scrubbing secrets). He went so far as to install cameras to remotely keep an eye on things in his London offices as well.

Whether Ruth Madoff recognized paranoia or other disturbing signs of her husband's misdeeds, she expressed surprise at his actions later.

"The man who committed this horrible fraud is not the man whom I have known for all these years," she said.

Former Penn State defensive coordinator Jerry Sandusky's wife, Dottie, backed him even as he was found guilty of sexually abusing 10 boys over a 15-year period.

Some of his alleged victims testified that his wife was in their home when the molestation occurred in the basement.

Dottie, a soft-spoken churchgoer and humanitarian, said in a letter to Judge John Cleland that she never heard or saw anything sexual going on.

Sandusky had been a beloved member of the community; he not only helped Penn State win two national championships but founded The Second Mile, a nonprofit organization for underprivileged

and at-risk children.

"Pretending has always been a part of me," Sandusky, who referred to himself as "The Great Pretender," tellingly wrote in his autobiography, "Touched: The Jerry Sandusky Story."

Madoff and Sandusky may have hidden their predilictions well. But even when a loved one poses a clear and present danger to society, relatives don't always intervene effectively.

In documents released in March, it was revealed that Arizona mass shooter Jared Loughner's behavior was so disturbing that his father confiscated his shotgun and disabled his car every night to keep him home in the months leading up to the rampage.

But when the signs are subtle, it's easy to miss them --sometimes it's even desirable from the point of view of the spouse.

Life partners have an investment in seeing one another through rose-colored glasses, Saltz says, especially if they have a child and are deeply invested in the marriage.

"It does take some complicity on the part of the person that doesn't know in order to not know. It's not necessarily conscious complicity," she said.

Brateman says denial absolutely comes into play.

"They may see some of these symptoms and not actually know what they mean," Brateman said. "They say, 'Oh, he/she is agitated' or 'in a bad mood,' but they don't ever imagine that that could mean something."

"In the aftermath of the Patriots' Day horror, we know that we never really knew Tamerlan Tsarnaev," said the Russell family's statement. "Our hearts are sickened by the knowledge of the horror he has inflicted."

Russell, whom her lawyer Amato DeLuca describes as "very distraught," might be dealing with the psychological impact of realizing her life with Tsarnaev was not normal. She may feel robbed of her belief system and sense of self, Brateman says.

Living a dual life is "the ultimate act of betrayal," Brateman says. "It doesn't just defy marriage, it defies humanity."